Prevent double-deletion of managed objects
Merge branch 'master' into coredata-issue

Conflicts:
	MantleTests/MTLManagedObjectAdapterSpec.m
	MantleTests/TestModel.xcdatamodeld/TestModel.xcdatamodel/contents
Merge branch 'master' into coredata-issue
Use temp variable to work around memory issue
Less branching
Merge pull request #120 from robb/coredata-issue

Prevent deleting `nil` in MTLManagedObjectAdapter